The first 16-channel transceive surface-coil array that conforms to the human head and operates at 298 MHz (7 T) is described. Individual coil elements were decoupled using circumferential shields around each element that extended orthogonally from the former. This decoupling method allowed elements to be constructed with arbitrary shape, size, and location to create a three-dimensional array. Radiofrequency shimming achieved a transmit-field uniformity of 20% over the whole brain and 14% over a single axial slice. During radiofrequency transmission, coil elements couple tightly to the head and reduce the amount of power necessary to achieve a mean 90°flip angle (660-ms and 480-ms pulse lengths were required for a 1-kW hard pulse when shimming over the whole brain and a single axial slice, respectively). During reception, the close proximity of coil elements to the head increases the signal-to-noise ratio in the periphery of the brain, most notably at the superior aspect of the head. The sensitivity profile of each element is localized beneath the respective shield. When combined with the achieved isolation between elements, this results in the capacity for low geometry factors during both transmit and receive: 1.04/1.06 (mean) and 1.25/1.54 (maximum) for 3-by-3 acceleration in the axial/sagittal plane. High cortical signal-tonoise ratio and parallel imaging performance make the conformal coil ideal for the study of high temporal and/or spatial cortical architecture and function.
Quantitative MRI techniques as well as methods such as blood oxygen level-dependent (BOLD) imaging and in vivo spectroscopy require stringent optimization of magnetic field homogeneity, particularly when using high main magnetic fields. Automated shimming approaches require a method of measuring the main magnetic field, B 0 , followed by adjusting the currents in resistive shim coils to maximize homogeneity. A robust automated shimming technique using arbitrary mapping acquisition parameters (RASTAMAP) using a 3D multiecho gradient echo sequence that measures B 0 with high precision was developed. Inherent compensation and postprocessing methods enable removal of artifacts due to hardware timing errors, gradient propagation delays, gradient amplifier asymmetry, and eddy currents. This allows field maps to be generated for any field of view, bandwidth, resolution, or acquisition orientation without custom tuning of sequence parameters. The homogeneity of the static magnetic field, B 0 , is critical for many fast, quantitative, and spectroscopic imaging techniques. The trend towards higher static magnetic fields, both clinically and experimentally, is tempered by the increased magnetic field distortions caused by susceptibility differences. One method to compensate for these distortions is using higher-order resistive shim coils. However, the practical optimization of numerous higher-order resistive shim currents is only possible with automated techniques. Manual shimming by monitoring integrated signal magnitude and line shape during repeated adjustment of individual shim currents according to operator judgment or some mathematical algorithm (1) requires prohibitive amounts of time. Iterative readjustment of shim currents is required due to shim coil cross-terms resulting from either imperfect coils or off-center localization. In addition, signal weighting due to variations in longitudinal and transverse relaxation rates can cause inappropriate spatial biasing of these FID envelope methods.Automated methods of improving homogeneity developed to alleviate these inherent difficulties fall into two classes, projection mapping (2,3) and volume mapping (4,5). Projection mapping is advantageous for its short acquisition time. However, it relies on the incorrect assumption that shim coil fields are always fully characterized by a minimal set of spherical harmonics. In general, projection mapping also involves localization techniques that make it ill-suited for disjoint regions such as in multivoxel spectroscopy. Volume field mapping eliminates the reliance on spherical harmonics by using full 3D maps of the magnetic field generated by the shim coils to determine the optimum current settings. Regions of signal void do not affect the shim current calculations, and arbitrary, potentially disjoint regions over which to shim can be specified. This additional flexibility comes at the expense of increased time, since chemical shift imaging (6,7) and phase mapping (8,9) techniques that have been proposed to create volume B 0 ma...
Estimates of the apparent transverse relaxation rate (R 2 * ) can be used to quantify important properties of biological tissue. Surprisingly, the mechanism of R 2 * dependence on tissue orientation is not well understood. The primary goal of this paper was to characterize orientation dependence of R 2 * in gray and white matter and relate it to independent measurements of two other susceptibility based parameters: the local Larmor frequency shift (f L ) and quantitative volume magnetic susceptibility (Δχ ). Through this comparative analysis we calculated scaling relations quantifying R 2 ′ (reversible contribution to the transverse relaxation rate from local field inhomogeneities) in a voxel given measurements of the local Larmor frequency shift. R 2 ′ is a measure of both perturber geometry and density and is related to tissue microstructure. Additionally, two methods (the Generalized Lorentzian model and iterative dipole inversion) for calculating Δχ were compared in gray and white matter. The value of Δχ derived from fitting the Generalized Lorentzian model was then connected to the observed R 2 * orientation dependence using image-registered optical density measurements from histochemical staining. Our results demonstrate that the R 2 * and f L of white and cortical gray matter are well described by a sinusoidal dependence on the orientation of the tissue and a linear dependence on the volume fraction of myelin in the tissue. In deep brain gray matter structures, where there is no obvious symmetry axis, R 2 * and f L have no orientation dependence but retain a linear dependence on tissue iron concentration and hence Δχ .MRI contrast mechanisms | grey matter | cellular architecture | relaxation times I n many neurological diseases such as multiple sclerosis, Alzheimer's, and Parkinson, and in conditions following traumatic brain injury, microstructural changes occur in gray and white matter (1-4). One method for quantifying these microstructural changes is the mapping of the effective transverse relaxation rate (R p 2 ). Along with the longitudinal relaxation rate (R 1 ) and transverse relaxation rate (R 2 ), R p 2 has been viewed as a fundamental MRI tissue parameter, affected by several factors including myelin content (5, 6), endogenous ferritin-based (Fe 3+ ) iron (7,8), tissue microstructure (6), and paramagnetic, blood deoxyhemoglobin (9). However, a number of recent studies have reported a somewhat surprising dependence of R p 2 on tissue orientation, at least in white matter (10-12). The purpose of this paper was to investigate the mechanisms that could contribute to this orientation dependence of R p 2 in both gray and white matter. Because R p 2 is influenced by magnetic field perturbations, we examined the role of local Larmor frequency shift (f L ) and quantitative magnetic susceptibility (Δχ), parameters that relate field and frequency. Through this analysis we identified unique scaling relations that relate R 2 ′ to the local Larmor frequency shift calculated after removal of macroscopic field ...
Blood-oxygenation-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) is currently the dominant technique for non-invasive investigation of brain functions. One of the challenges with BOLD fMRI, particularly at high fields, is compensation for the effects of spatiotemporally varying magnetic field inhomogeneities (ΔB0) caused by normal subject respiration, and in some studies, movement of the subject during the scan to perform tasks related to the functional paradigm. The presence of ΔB0 during data acquisition distorts reconstructed images and introduces extraneous fluctuations in the fMRI time series that decrease the BOLD contrast-to-noise ratio. Optimization of the fMRI data-processing pipeline to compensate for geometric distortions is of paramount importance to ensure high quality of fMRI data. To investigate ΔB0 caused by subject movement, echo-planar imaging scans were collected with and without concurrent motion of a phantom arm. The phantom arm was constructed and moved by the experimenter to emulate forearm motions while subjects remained still and observed a visual stimulation paradigm. These data were then subjected to eight different combinations of preprocessing steps. The best preprocessing pipeline included navigator correction, a complex phase regressor, and spatial smoothing. The synergy between navigator correction and phase regression reduced geometric distortions better than either step in isolation, and preconditioned the data to make them more amenable to the benefits of spatial smoothing. The combination of these steps provided a 10% increase in t-statistics compared to only navigator correction and spatial smoothing, and reduced the noise and false activations in regions where no legitimate effects would occur.
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